1. Field of the Invention
The present invention relates to a method for the hydrolysis of a lower alkyl ester to produce a carboxylic acid and corresponding alkyl ether using reactive distillation. The present invention is particularly useful for hydrolysis of methyl acetate to form dimethyl ether and acetic acid.
2. Background of the Invention
Methyl acetate is produced in great amounts as a byproduct from purified terephthalic acid (PTA) plants and polyvinyl alcohol (PVOH) plants. Since methyl acetate is a less valuable solvent, methyl acetate is sold at a low price or hydrolyzed to methanol and acetic acid, which are more valuable, by a hydrolysis reaction followed by conventional distillation processes. Conventional distillation after reaction, however, has some drawbacks. First of all, this complex process has low reactor yield due to the small equilibrium constant which governs the hydrolysis reaction. This requires a large recycle stream of methyl acetate.
The methyl acetate mixtures from both PVA processes and from PTA processes have azeotropic mixtures of methyl acetate/methanol and methyl acetate/water as described in Table I. It is possible to improve methyl acetate hydrolysis by increasing the amount of water in the feed, since the reaction is an equilibrium reaction. However, any water added to the system must be removed later making the overall energy requirements of the system increase as the amount of water is increased. The large methyl acetate recycle along with the separations of methanol/water and water/acetic acid result in a large process heat requirement.
Another drawback is high fixed costs and high operating costs resulting from separated hydrolysis reactors and multiple distillation towers including extractive distillation columns and dehydration columns. Accordingly, the high capital and energy costs expected for the conventional process indicate that a new process for the hydrolysis of methyl acetate would be highly desirable.
To improve the conversion of methyl acetate to acetic acid and methanol and the separation of these materials from the methyl acetate feed stream and water, U.S. Pat. No. 5,113,015 issued to Palmer et al. on May 12, 1992, discloses a reactive distillation process whereby methyl acetate is converted to acetic acid and methanol. The process includes contacting the methyl acetate with water in the presence of a catalyst packing material comprising a hydrolysis catalyst and a solvent comprising acetic acid to produce methanol and acetic acid, and coextensively, separating the methanol from the acetic acid.
Reactive distillation is a method to integrate reaction and distillation in the same column. Although reactive distillation has been known since the 1920s, most of the reaction and distillation processes have been independently operated. The advantages of the reactive distillation process for methyl acetate hydrolysis are more attractive than those of the conventional process, because integrating reaction and distillation technology in the same column reduces capital and operating costs greatly. Most hydrolysis or esterification reactions are limited by chemical equilibrium, but the reactive distillation process can shift the equilibrium reaction forward by removing the products, acetic acid and methanol, continuously from the reactants, methyl acetate and water. In cases where an azeotropic mixture is formed in the column, reactive distillation can break azeotropic compositions resulting in an increase in reaction yield and selectivity.
U.S. Pat. No. 5,770,770 issued to Kim et al. on Jun. 23, 1998 discloses a reactive distillation process for producing acetic acid and methanol as hydrolysis products from a byproduct composition containing more than 50% of methyl acetate. The process includes (a) hydrolyzing a mixture containing methyl acetate to acetic acid and methanol in a reaction zone, wherein ion exchange resin packing is present in the reaction zone as a catalyst, water is supplied downwardly to the ion exchange resin packing and the methyl acetate mixture is supplied upwardly to the ion exchange resin packing, (b) collecting unreacted methyl acetate and water vapor occupying the upper part of the reaction zone, condensing and returning them to the reaction zone, and (c) at the same time as step (b), collecting a reaction mixture from step (a) occupying the lower part of the reaction zone, separating the reaction mixture into the hydrolysis products and impurities by reboiling, returning the impurities to the reaction zone and recovering the hydrolysis products.
A problem with the aforementioned reactive distillation processes in converting methyl acetate to acetic acid and methanol is that large feed ratios of water/methyl acetate are required. This large amount of water in the bottoms product has to be separated from the desired acetic acid in one of the separation columns which results in increased heat requirements and capital costs.
Another problem with the aforementioned reactive distillation processes in converting methyl acetate to acetic acid and methanol is that large amounts of unreacted methyl acetate and water in the distillate must be condensed and returned to the reaction portion of the distillation column as well as reboiling a substantial amount of water from the bottoms. This substantially adds to the cost of energy and equipment.
Accordingly, there still exists a need for an improved process for the hydrolysis of methyl acetate.
As described above, a low equilibrium constant (Keq≈0.14) and the presence of interfering azeotropes inhibit the hydrolysis of methyl acetate via conventional processes. Surprisingly, it has now been found that the problems previously associated with the hydrolysis of methyl acetate to form acetic acid and methanol can be overcome by preferentially driving the hydrolysis of the alkyl ester to form a carboxylic acid and an ether. In the case of methyl acetate, the products are acetic acid (bottoms) and dimethyl ether (distillate), which can be produced by the dehydration of methanol by using an alumina catalyst disclosed in U.S. Pat. No. 4,595,785, the entire disclosure of which is incorporated herein by reference. Accordingly, the process of the present invention is for hydrolyzing an alkyl ester to produce a carboxylic acid and an ether. The process includes the steps of introducing into a reaction zone of a reactive distillation column a feed containing the alkyl ester wherein the reaction zone of the distillation column is at a temperature and pressure that preferentially produces an ether from the ester; and recovering the ether.
From the description that follows, one skilled in the art will understand that, while the invention focuses on the hydrolysis of methyl acetate, the invention may be useful for the hydrolysis of other esters in which the corresponding ether has a lower boiling point than other components and preferably the azeotropes within the system.
It is an object of the present invention to provide a method for the hydrolysis of an alkyl ester to form a carboxylic acid and an ether.
It is another object of the present invention to provide a method for the hydrolysis of methyl acetate to form dimethyl ether and acetic acid.